1. Field of the Invention
Our invention relates to an environmentally friendly method of heating a furnace using improved gas-fired burners, particularly the type found in industrial furnaces. More specifically, our improved heating process uses a burner design that produces extremely low levels of NOx.
2. The Prior Art
Industrial gas burners are designed to generate heat and produce high temperatures, typically in the range of from 1,200xc2x0 F. to 2,300xc2x0 F. At such temperatures, thermal nitrogen oxides (NOx) can form as gaseous byproducts of the combustion of air and the hydrocarbon gas used as the fuel in the burners. These NOx byproducts are a major source of air pollution and governmental authorities have instituted strict environmental regulations limiting the amount of NOx gases that can be emitted into the atmosphere. The art has recognized that reducing the peak flame temperature of industrial burners can minimize NOx formation. As taught in U.S. Pat. No. 5,073,105, lower flame temperatures may be achieved by recirculating a small portion of exhaust gases (also known as furnace or flue gases) into the combustion zone to mix with the hydrocarbon fuel and combustion air. Specifically, the recirculated furnace gases are mixed with hydrocarbon fuel gas followed by mixing with the combustion air before combustion. U.S. Pat. Nos. 6,007,325 and 5,984,665 describe a burner design that has three flame regions, where the first region is formed using a pre-mix burner tip to combust a lean fuel-air mixture. In addition to the pre-mix burner tip, these designs also use recirculated furnace gases. Although prior burner designs may have recirculated a small or limited amount of furnace gases, the art has not fully recognized the importance that recirculated furnace gases have on reducing NOx formation. In particular, there is very little, if any, teaching suggesting that dramatically increased amounts of recirculated furnace gases will greatly reduce NOx levels without adversely affecting burner performance. Indeed, prior to our invention it was believed that increasing the amount of recirculated furnace gases would, at minimum, cause flame instability. Contrary to that accepted view, we have found that significantly increasing the amount of furnace gases circulated back to the burner did not affect flame stability. Instead, the increased flow of furnace gases greatly lowered the amount of NOx gases formed to levels of less than 10 ppm. These low levels were obtained without the use of a complicated pre-mix burner apparatus.
Accordingly, an object of our invention is to provide a method for heating an industrial furnace with an improved burner design that has greatly reduced NOx emissions.
Another object of our invention is to provide an improved burner design that recirculates significantly more furnace gases than prior designs in order to prevent excessive flame temperatures and thus greatly reduce the formation of nitrogen oxides.
Yet another object of our invention is to provide a process of heating a furnace where furnace gas is recirculated back to the burner through recirculation ports having at least 5 sq. in. of total cross-sectional port exit area per 1 million (MM) BTU/hr of heat generated.
As stated, our invention is directed to a process for heating industrial furnaces using an improved gas fired burner design. Our process and improved burner design generates less than 10 ppm by volume of NOx. Such low levels of nitrogen oxides will greatly reduce the air pollutants currently being emitted by existing industrial furnaces using prior art burners. Our improved burner design produces a cooler flame and thus lowers NOx formation. These benefits are possible because of modifications that we have made to the tile design, recirculation port configuration, and gas tip configuration and placement. By use of the term xe2x80x9crecirculation portxe2x80x9d we mean any opening or channel through the burner block that is designed to channel a mixture of fuel gas and furnace gases into the primary combustion zone. Each burner of our invention can be characterized by a xe2x80x9ctotal recirculation port area,xe2x80x9d which we define as the sum of the individual cross-sectional areas of each recirculation port exit opening. The xe2x80x9cexit openingxe2x80x9d is the port opening that is adjacent and in communication with the primary combustion zone. The xe2x80x9centrance openingxe2x80x9d is the port opening adjacent to the primary fuel tip and where the recirculation furnace gases enter the recirculation port. The cross-sectional area of the port exit opening is measured at the outermost edge of the exit opening.
One of the most significant improvements in our new burner design is the ability to recirculate a large amount of furnace gases back to the burner for mixing with the fuel gas prior to combustion, when compared to prior art designs. In some cases we are able to recirculate a significantly greater amount of furnace gases as compared to prior art designs. Surprisingly and unexpectedly we have found that recirculating such a large amount of the furnace gases dramatically reduces the amount of NOx gases formed without causing flame instability. Increasing the amount of furnace gases returned to the burner improves the mixing and dispersion of the fuel gas prior to combusting the fuel with air. By using the relatively inert furnace gases to disperse the fuel gas prior to mixing with the combustion air in the primary combustion zone, a cooler burning flame is achieved. A cooler flame in turn greatly reduces the undesirable formation of NOx. In addition to lowering the NOx formed, we also found that increased recirculation of furnace gas did not adversely affect flame stability. Moreover, our improved burner design allowed us to eliminate the need for a lean pre-mix burner tip of the kind described and used in the prior art.
The increase in furnace gas recirculation is achieved in part by increasing the available cross-sectional area of the recirculation ports. The recirculation ports resemble large holes or tunnels, which are located around the circumference of the burner tile (also known as the burner block) and which allow the furnace gases to pass from the outside of the burner into the primary combustion zone located in the center of the burner. Typical prior art designs have no more than 4.8 in2 of total recirculation port area per million (MM) BTU/hr, whereas our design has increased the total recirculation port area to at least 5 in2 per MM BTU per hr. Our preferred range is from at least 5 in2 per MM BTU per hr to about 12.5 in2 per MM BTU per hr. Accordingly, for a 1 MM BTU burner design the total recirculation port area would be 5 in2. Likewise, for an 8 MM BTU burner, the recirculation port area would be 40 in2. This increase in recirculation port area can be achieved in a number of ways, including increasing the total number of ports or increasing the physical size of the existing number of ports, compared to designs currently in use. Our preferred design increases the number of ports by 1.5 to 2.0 times the number used in prior devices and/or modifies the shape of the port. In our most preferred design, each recirculation port exit opening is at least 0.625 in2 per MM BTU/hr of heat generated by the burner. As those skilled in the art will appreciate, calculating the heat duty (or heat generation) of a burner is accomplished using well known engineering principles and is a function of fuel type, fuel tip area and fuel pressure. More typically, one can determine the heat generation of a given burner by consulting the manufacturer""s specification, which is usually equivalent to the specification set by the customer. The heat generation referred to in this application is the total heat generation and is based on both the primary and secondary fuel tips. In our preferred design, 15 to 45% of the total heat generation is due to the primary fuel tips. Accordingly, using the primary fuel heat generation as a basis, our invention would have a range of total available cross-sectional area of from about 5 in2 per 150,000 BTU per hr to about 5 in2 per 450,000 BTU per hr.
As mentioned, we have also found that it is highly advantageous to change the shape of the recirculation ports by having the opening or entrance of the port on the outside surface of the burner tile larger than the exit opening on the inside surface of the burner tile. Also, instead of having the port entrance with sharp corners, we have rounded at least part of the entrance opening. This tapered port and contoured inlet configuration of the port entrance enhances a venturi effect that in turn increases the quantity of furnace gases drawn into the recirculation ports. Moreover, by increasing the venturi effect there is an improvement in the mixing of the fuel gas and recirculated furnace gas. In a preferred design the edges of the port entrance openings are rounded or curved in shape and have a radius of at least xc2xd inch. To further enhance the venturi effect we position the primary fuel gas tip outside of the recirculation port entrance. This configuration further assists in drawing the furnace gases and fuel gas into the port where they are intimately mixed and dispersed prior to exiting into the primary combustion zone of the burner. This well mixed fuel/furnace gas mixture is then mixed with air and is combusted in the primary combustion zone. By dispersing the fuel gas in the inert furnace gases in the recirculation port prior to mixing with the combustion air greatly reduces the chance that high peak flame temperatures will occur and thus reduces the possibility that high levels of NOx will form.
Another improvement found in a preferred embodiment of our burner design is an increased tile wall thickness. Typical prior art designs have tile thickness of 3 inches or less. The thicker tile wall increases the length of the recirculation port wall, thus effectively increasing the residence time available for the fuel and furnace gases to mix. The thickness of the tile wall is measured along the centerline of the recirculation ports. A preferred thickness is greater than 3 inches, more preferably 5 xc2xd inches or more. Another way to increase the residence time is to increase the distance from the primary fuel tip orifice to the exit opening of the recirculation port. In prior art burners this distance is a maximum of about 4 inches. We have found that distances greater than 4 inches will be beneficial. The increased residence time allows the fuel gas to more completely disperse in the recirculated furnace gases prior to exiting the recirculation port and entering the primary combustion zone. In addition, the increased length of the ports reduces the tendency of air migrating into the port prior to combustion with the fuel/furnace gas mixture in the primary combustion zone. The primary fuel tips used to inject fuel into the recirculation ports is located on a fuel pipe connected to a fuel gas manifold. In some cases it is advantageous to combine the primary and secondary fuel tips on a single fuel pipe. This is referred to as doubled drilled tips or a combination of secondary and primary tips, where the primary fuel tip is drilled into the lower portion of the pipe and the secondary fuel tip is drilled into the upper portion of the fuel pipe. Another design uses separate fuel pipes for the primary and secondary fuel tips.